1. Field of the Invention
The invention relates generally to an optical waveguide used as a device for optical signal transmission.
2. Background Art
In order to lower connection losses with an optical fiber, an optical waveguide ideally is produced with a core having a rectangular cross-sectional shape. However, when the optical waveguide is reproduced using a stamper, cross-sectional shape of the resultant core is tapered (trapezoidal) due to release tapering and the like due to molding by the stamper (patent citation no. 1 and the like). FIG. 1 is a schematic titled-perspective view showing one example of an optical waveguide having a core cross-sectional shape which is tapered. The optical waveguide 10 comprises an upper clad layer 14 stacked on a lower clad layer 13, and sandwiched between a lower glass substrate 11 and an upper glass substrate 12, and a core 16 filling a core trench 15 disposed in the upper face of the lower clad layer 13.
The lower clad layer 13 of this type of optical waveguide 10 is produced, for example, by the stamping method. According to this stamping method, an ultraviolet light curing type resin is poured on the glass substrate 11, the ultraviolet light curing type resin is pressed by a stamper and is spread out between the stamper and the substrate, and the lower clad layer 13 is molded by causing the ultraviolet light curing type resin to cure by exposure to ultraviolet light. Although during such production the core trench 15 is formed in the upper face of the lower clad layer 13 due to a convex shape possessed by the stamper, due to the imparting of a taper tilted with respect to the direction of mold separation at both side faces of the convex shape for ready mold release of the stamper, a taper is formed along the depth direction at both side faces of the core trench 15. Thereafter the core 16 is formed by filling the core trench 15 molded in the upper face of the lower clad layer 13 with a core material. Thereafter the upper clad layer 14 is formed between the lower clad layer 13 and the glass substrate 12, and the core 16 is sealed between the lower clad layer 13 and the upper clad layer 14. Thus the interior of the core trench 15 is filed with the core material, and as viewed in cross section perpendicular to the core lengthwise direction, a taper is imparted along the depth direction at both side faces of the core 16.
Moreover, in order to make the mold readily release from the lower clad layer formed from resin within the mold during molding of the lower clad layer by irradiation molding, a taper is imparted to the core trench along the depth direction. Moreover, even when the core is formed by deposition of the core material on the lower clad layer utilizing a semiconductor manufacturing process, a taper is produced in the core after etching due to spreading of light in the core material layer depth-wise direction during exposure. Thus, for either of these manufacturing methods, the core cross-sectional shape is tapered due to the imparting of the taper to the core.
When a taper is imparted in this manner along the height direction or depth direction as viewed in a cross section perpendicular to the length-wise direction of the core, width of the core cross section varies according to height above the lower face of the core. Thus, the effective refractive index of the part of narrow width of the core cross section is low in comparison to the effective refractive index of the part of wide width of the core cross section. Due to a lowering of the difference between this effective refractive index and refractive index of the lower clad layer or the upper clad layer, light confinement within the core becomes weaker at the part of narrow core cross sectional width in comparison the to the part of wide core cross sectional width. Therefore, when a taper is imparted to the core cross section, the height-direction mode profile around the center of the core becomes asymmetric due to the variation of width of the core cross section.
FIG. 2(a) shows the lower clad layer 13, and the upper clad layer 14, and the core 16 having a tapered cross section such that core width increases at the upper face and narrows at the lower face. Within FIG. 2(b), variation of refractive index along the height direction within this core 16 is indicated above and below the center of the cross section of the core 16. Within FIG. 2(c), the mode profile of optical signal transmission through the interior of this core 16 is indicated by the solid line. The vertical axis of FIG. 2(c) indicates height as measured from the center in the height direction of the cross section of the core 16, and the horizontal axis indicates intensity of the optical signal transmitted within the core 16. Moreover, the curve indicated by the broken line within FIG. 2(c) shows an ideal mode profile of a core having a rectangular cross-sectional shape. The ideal mode profile indicated by the broken line within FIG. 2(c) is centrally symmetric along the height direction of core cross section. In contrast, in the case of the tapered core cross section, leakage of light to the lower clad layer 13 at the lower face of the core 16 becomes high, and this mode profile becomes asymmetric.
Due to vertical direction asymmetry of the core mode profile of a conventional optical waveguide having a core of tapered cross section in this manner, there is a high loss of light resulting from connection with an optical fiber having a vertically symmetric mode profile of light transmitted through the core of the optical fiber.
[Patent Citation 1] Japan Publication of Unexamined Patent Application No. 2003-240991.